MAJOR OBJECTIVES: 1) Determine O2 saturation of myoglobin, homogeneity of intracellular PO2, and O2 gradients in microvessels of skeletal muscle under various conditions of VO2 and O2 supply. 2) Determine the function of myoglobin during the transition from rest to exercise, and during steady work. 3) Study controls of blood flow, capillary density and capillary recruitment. 4) By integrating 1-3 above, understand the mechanisms of circulation-metabolism coupling in muscle. 5) Determine intracellular PO2 in normal and ischemic myocardium, with a view to reconciling the present disparity between biochemical, anatomical and physiological definitions of a "border zone". METHODS: O2 saturation of myoglobin and calculated PO2 in single cells and parts of cells will be measured in dog gracilis muscles freeze-clamped at rest and during exercise. Spatial resolution of the reflectance microspectrophotometer is 5 microns. Since freezing arrests chemical reaction, measurements on a large population of cells may be regarded as having been made simultaneously. Thus the purely spatial distribution of intracellular PO2 can be determined independent of time, and undistorted by the mechanical effects of O2 electrodes. The number and location of erythrocyte-containing capillaries and microvessels around each muscle-fiber studied will be determined, and the O2 saturation of hemoglobin in these vessels will be measured. Results will be correlated with flow and VO2 of the whole muscle, and with parallel studies of the neural and metabolic determinants of exercise vasodilation. In the final 2 years of the project we will extend the method to the epicardium of the canine heart. (The subendocardium cannot yet be frozen rapidly enough to permit spectrophotometry). Homogeneity of cell PO2 and local capillary density will be evaluated in normal myocardium and in the "border zones" around areas of ischemia.